In the course of medical treatment, it is often desirable to measure the blood pressure of a patient. In the past this has been done by means of external pressure transducers. More recently, blood pressure has been measured by the placement of a pressure transducer directly inside a blood vessel of the patient or inside a catheter which is placed inside the patient's blood vessel. The placement of the transducer inside a blood vessel enables the observation of blood pressure waveforms having high dynamic fidelity.
All known blood pressure transducers experience some drift in their outputs at zero pressure. They must therefore be rezeroed to atmospheric pressure periodically. Rezeroing is typically accomplished by a control on the monitor to which the transducer is connected which adjusts the measured output signal to correct for the offset observed at zero (atmospheric) pressure.
Rezeroing a transducer placed inside the blood vessel presents unique difficulties, since the temporary removal of the transducer from the blood vessel to vent the tip of the atmosphere is a difficult procedure which presents risks to the patient when conventional rezeroing methods are used.
An in-vivo pressure transducer typically comprises a Wheatstone bridge mounted at least partially on a diaphragm. Rezeroing involves the equalization of the pressure on the bottom side of the diaphragm with that on the top side of the diaphragm. A method and apparatus for rezeroing a pressure transducer in-vivo are described in U.S. Pat. No. 5,203,340, which is incorporated by reference. Using the method and apparatus described and claimed in U.S. Pat. No. 5,203,340, the transducer can be rezeroed without removing the transducer from the blood vessel.
The in-vivo rezeroing of a pressure transducer has a peculiar limitation. The blood pressure typically seen by a pressure transducer in-vivo is dynamic and pulsatile. It takes a certain length of time for a pressure pulse wave to travel from the tip of the catheter, in which the transducer resides and at which the pressure wave impinges upon one side of the pressure transducer diaphragm, along the lumen of the catheter, through the rezeroing device and back to the other side of the pressure transducer diaphragm to accomplish the rezeroing. Thus, while the mean pressure on the diaphragm may be zero, a pulse artifact remains causing fluctuations of pressure about the mean.
Typical pressure monitors are adapted to detect pressure based on hydraulic coupling between the transducer and blood vessel. In such cases rezeroing is accomplished by means of an external stop-cock which can be used to establish an atmospheric datum pressure without producing any pulsations. Some monitors reject pressure pulses as being uncharacteristic of pressure transducers which are in the process of being rezeroed. Other monitors do not reject pulses but average the signals. They may not average the signals over a sufficiently long period of time to prevent integration errors. Either of these monitor characteristics will seriously limit the use of in-vivo pressure transducers using the method of rezeroing described above. There is therefore a need for a means by which an in-vivo transducer can be adapted for simple and effective rezeroing when used with typical pressure monitors.